Modification of polymer surfaces has often been employed for cleaning purposes, to induce crosslinking, to increase wettability, to prepare the surface for adhesion, and so on. The surface properties of the polymer are effected depending upon the combination of the polymeric chemical structure and method of surface modification utilized.
Polytetrafluoroethylene (PTFE) as a material is favored for many uses because it is relatively inert and has low adhesion and low friction properties. However, these very same properties create problems when it is desired to bond this polymer to diverse surfaces, making it necessary to first modify the PTFE surface. This has been accomplished in the past by transforming the surface into an air stable amorphous phase of carbon by reduction with amalgams of alkali metals such as lithium.
L. Cervinka et al., in "Study and Analysis of Structural Properties of Non-crystalline Carbon Phase Prepared from PTFE", Philosophical Magazine B, Vol. 51 No. 6, 603-633 (1985), proposed a model structure of such a modification resulting in amorphous carbon that consists of graphite-like regions with a relatively well-ordered layer of graphitic carbon, separated by small, dispersed diamond-like carbon regions. Modification of the PTFE in this manner affords adhesion via Van der Waals forces to certain other conventional polymers.
Other surface modifications of PTFE known to the art include treatments with alkali metals, solutions of alkali metal-naphthalides, and liquid sodium-ammonia treatments, as well as direct electrochemical reduction. These treatments prepare the surface for improved adhesion to other materials via the formation of air stable carbonaceous surface layers composed largely of graphitic and amorphous carbon.
Solbue et al., Chemical Abstracts 76 (20):113814f, 1971, report a graft copolymerization of styrene onto Teflon.RTM. (PTFE) using gamma radiation. This grafting might possibly be attributed to x-ray generation of free radicals on the PTFE surface which initiate polymerization of styrene. The investigators found the thermal behavior of styrene-grafted Teflon.RTM. (PTFE) to be similiar to that of a polystyrene-Teflon.RTM. (PTFE) blend, however, workability of PTFE was not improved.
Reduction of PTFE surfaces has also been effected through the use of a dianion of benzoin (benzil dianion) in a suitable solvent yielding a gold-colored, air-sensitive surface film. This is in contrast to the air stable, black carbonaceous surfaces produced by the chemical methods described above. Costello and McCarthy, Macromolecules, vol. 18, (Dec. 1985) pp. 2087-88, report the above reaction as converting the surface of PTFE to a metallic gold-colored, air-sensitive, nearly totally carbonaceous material. No uses were reported for this material, however, they explored the electrical conductivity of the modified surface and found it to be low until acceptor-doped with iodine.